The present invention relates to a support structure for the foot. More particularly, the present invention relates to a shoe construction formed of sling straps which may be anchored into the lateral border of the top of the outsole of a shoe under the lower, lateral column of the foot. The straps are then carried over the top of the midfoot and allowed to fan out with attachment to the outsole under the upper, medial column of the foot so as to optimally support the upper column when the foot is loaded.
In an attempt to understand the foot as a system, the various parameters which affect the function of the foot have been studied, particularly with regard to a weight bearing foot. The practical need for such knowledge lies in the fact that a true structural model of the foot is capable of providing a prediction of gait and the effects of a shoe on gait. By knowing, in advance, how a shoe would affect the performance of an athlete, for example, optimum shoes could be designed without the usual "cut and try" method of standard shoe development.
The traditional model of the foot provides for a one column, two-axis model which maintains that the foot under load is a rigid structure with a talocrural (ankle) axis and an apparent subtalar axis. The front of the foot is relatively rigid, but with only a multitude of small bone movements about the midtarses axes. The average direction of the effective axis under the ankle, called the subtalar axis, is said to be 42 degrees vertical and 16 degrees horizontal to the midline of the body, as measured by Inman, V. T., The Joints of the Ankle, The Williams & Wilkins Co., Baltimore, 1976. However, this theory does not hold up with regard to a weight bearing or loaded foot since, if the force due to body weight were to act on the single traditional subtalar axis, the foot would collapse mechanically.
It has now been determined that the foot is comprised of two columns and three axes. The lower, lateral column is basically a rigid base comprised of the Calcaneus, Cuboid, and the fourth and fifth metatarsals. The remainder of the foot, which is comprised of the navicular, the first, second and third cuneiforms and the first, second and third metatarsals, emanates from the talus at the talonavicular interface swinging in combination with the lower column inversion/eversion actions in what may be called the `subtalar joint axis`. But this articulation of what is called the upper foot column is only secondary to the true foot mechanism. The primary mechanical loading interface is on the lower, lateral column at the rear of the talus onto the calcaneus, the posterior talocalcaneal facet.
It has also been determined that the foot operates differently under load than when it is passively manipulated such as a doctor would do in the office. This distinction helps to explain previous misconceptions as to how the foot works under load.
This new understanding has yielded a new structural model of the foot which has two separate columns, wrapped together with fascia, and three nearly orthogonal axes. The three axes are: (1) the talocrural (ankle) axis; (2) the talocalcaneal axis (formed at the facet between the talus and the calcaneus); and (3) the talonavicular axis (formed at the facet between the talus and the navicular bones).
The transverse support sling of the present invention represents a novel structure which is based on providing maximum foot support in the midfoot region in accordance with the two column foot model. The advantages of the transverse support sling of the present invention over the prior art structures include the act that the position of the foot relative to the shoe is maintained because the transverse sling is anchored into the outsole. This is accomplished without pulling or pushing the medial longitudinal arch to the shoe.
The transverse sling straps of the present invention are positioned relative to the outsole of the shoe so as to coincide with certain anatomical landmarks of the foot. Thus the sling straps include anterior and posterior straps on the lateral side of the shoe and anterior and posterior straps on the medial side of the shoe. One or more additional straps are spaced between the anterior and posterior straps on each side of the outsole. The straps are so positioned that the anterior lateral strap is posterior to the fifth metatarsal head. The posterior lateral strap passes across the foot proximate and adjacent the calcaneal-cuboid joint. The anterior medial strap must be directed posterior to the first metatarsal head. The posterior medial strap must be directed posteriorly, after passing proximate and adjacent the navicular protuberance.
The sling straps may be mounted in respective medial and lateral slots provided in the outsole. Alternatively, an end portion of each of the sling straps may be adhered directly to the upper surface of the outsole without the use of slots. The straps are each capable of serving as separate and independent lines of force which are necessary to provide the proper support for the midfoot region of the foot.
The medial slot of the outsole is in the form of an arcuate shape which lies beneath three anatomical points of the foot: (1) the posterior edge of the first metatarsal head; (2) the second or third cuneiform, preferably the third cuneiform; and (3) the medial side of the calcaneus. The straps are free of attachment to the outsole medially inwardly toward the medial edge of the outsole from the arcuate shape defined by the three anatomical points. It is noted that a smooth arcuate shape is only relevant to a smooth groove in the outsole, whereas individually anchor points would align to the direction of the strap.
The lateral slot of the outsole lies under the lower column of the foot throughout the length of the slot. This slot which is generally linear thus extends from the posterior edge of the fifth metatarsal head to a position proximate and adjacent the calcaneal-cuboid joint.
By the present invention, a larger percentage of the population can be properly fitted into a given length shoe, thereby reducing factory inventory. In addition, the sling dimensions and forces must be maintained until a manual readjustment is made.
Accordingly, it is a primary object of the present invention to anatomically enhance the structural support of the foot according to the two column, three axis foot model.
It is another object of the invention to provide a support sling construction with attachment to the shoe outsole such that adjustment is completely independent of rearfoot and forefoot fit.
It is a further object of the invention to provide a support sling construction which is adjustable to custom fit the midfoot for ninety percent of the statistical range of girths for a given foot length.
An additional object of the invention is to provide a transverse support sling in which the sling dimensions will remain unchanged after initial adjustment.
It is a further object of the invention to provide efficient, repeatable, ergonomic girthing adjustment, due to relatively inextensible sling straps .